NEW HAVEN, Conn. — Criminal charges won’t be filed in a Christmas-morning house fire that killed three girls and their grandparents and was blamed on a bag of discarded fireplace ashes, a Connecticut prosecutor said yesterday.

Stamford State’s Attorney David Cohen said yesterday that precautions were taken to prevent a fire and that while they were insufficient, they didn’t rise to the level of criminal negligence.

The girls’ parents, who are divorced, have filed notices that they plan to sue Stamford, accusing city officials of intentionally destroying evidence when they demolished the home a day after the fire.

Cohen said that “regrettably” the house was demolished and that, in the future, the police and prosecutor should be consulted before any demolition is carried out. Cohen said the investigation “was hampered to some degree” by city officials because of the demolition.

It was impossible to physically inspect the remains of the house to determine whether any working smoke detectors had been present, Cohen said, noting debris was removed before such an inspection could be performed.

Cohen said Borcina cleaned out the fireplace at about 3:30 a.m., shoveling the ash into a paper bag. Borcina said he smoothed out the ashes with his hand and that allayed any concerns Madonna Badger had about live embers being present, Cohen said.

The bag of ashes was then placed in a plastic storage box that was deposited just inside the exterior door in the mudroom, Cohen said.

“It is my opinion that there is insufficient evidence to establish that either Mrs. Badger or Mr. Borcina were aware of and consciously disregarded a risk that there was a possible live ember in the ash that could result in a catastrophic fire,” Cohen wrote.